1. Field of the Invention
The invention relates to a transmission system comprising a transmitter for transmitting digital signals by block-wise encoded modulation in accordance with points of a modulation pattern referred to as a "constellation", and a receiver having a decoder provided with first decoding means (45.sub.1 -45.sub.M) capable of generating bit erasures signifying bit sequence errors and second decoding means (44.sub.1 -44.sub.M1) capable of correcting bit erasures and bit sequence errors.
The invention more particularly relates to the transmission of digital television signals, for example high definition television signals, via a satellite channel or over microwave links. It may also relate to the transmission of digital sound signals by mobile radio, or transmission of digital data for storing, for example, on a compact disc, or by a digital tape recorder. In all these cases it is necessary to effect at the source a preliminary bit rate reduction by means of a source encoder, and on reception to reestablish the original bit rate by means of a source decoder. In those cases in which digital data is transmitted between two digital processing units, for example two calculators, bit rate reduction using a source encoder is not required.
Since the most representative use of the invention in high-definition television (HDTV), for which the problems of bit rate reduction are the most extensive, the present application is described with reference to this use but without any limitations as to other suitable uses.
2. Description of the Related Art
Digitizing of High-Definition Television signals (HDTV) generates a total gross data rate of the order of 800 Mbit/sec. The transmission of data at such a rate cannot be realized economically over existing transmission channels. Several coding techniques for reducing the data rate (source coding) have been developed employing coding algorithms which can be evaluated in terms of their data rate "reduction factor" and the quality of the picture obtained after decoding. The more the redundancy of the signal is reduced, the more significant is the actually transmitted information. Transmission errors which can easily be corrected if the information components are redundant, have increasingly serious consequences as the data rate reduction factor increases.
Consequently, the transmission of HDTV digital signals requires judicious error protection. So as to avoid the effects of transmission errors from being noticeable on a display screen, the error rate in the line scanning direction must be less than 10.sup.-11.
The channel which is used for "Direct Broadcasting by Satellite" (DBS) is characterized by:
a bandwidth of 27 MHz; PA1 a low power (above all for the link from the satellite) and the presence of much noise, which is considered to be additive, white and gaussian; and PA1 non-linear distortions. PA1 detection means, validated by the stages preceding it, for detecting the received points PR in the partition level of the said one stage, PA1 internal decoding means for effecting internal decoding by generating estimated inner code words of the internal code, and effecting erasure of all estimated inner code words for which the estimation decision is ambiguous, PA1 external decoding means for effecting an external decoding and which corrects the erasures and errors of the said estimated inner code words, the outputs of the external decoding means being encoded by coders which energize the detection means of the subsequent stages. PA1 calculate for each junction point of the trellis the accumulated distances (D1, D2) on the possible paths of the trellis distributed over the preceding decoded points, PA1 determine the path having the lowest accumulated distances, PA1 assign to the bit b.sup.p, corresponding to the said junction point a bit of the same significance p as that of b.sup.p.sub.1 or b.sup.p.sub.2 of the bit assignment of the point PT1 or PT2 of the last distance (D1, D2) of the accumulation, and when two accumulations for the same junction point are separated by a distance less than a predetermined small distance, adjust the internal code word assembly to the erasure state.
The routing of digital signals of HDTV via such a channel requires source coding having a considerable compression ratio as well as digital modulation having high spectral efficiency.
Coding techniques based, for example, on an orthogonal transform, can reduce the data rate by a factor higher than 10 whilst still ensuring a good quality of the recovered image. This leads to a binary transmission rate of the order of 60 to 70 Mbits/s. However, the transmission of such signals via a satellite channel requires digital modulation having a spectral efficiency reaching 2.7 bits/s/Hz.
It is further necessary to effect channel coding to protect the transmission from channel imperfections. Conventional channel coding and modulation techniques have proved to be inadequate to satisfy fully the requirements for correct transmission (in these techniques, the coding function is considered to be an entity which is independent of the modulation function). However, these coding techniques are significantly improved with the aid of the coding technique proposed by G. UNGERBROECK in the article: "Channel coding with multilevel/phase signals", published in IEEE Transactions on Information Theory vol. IT-28 no 1, January 1982, pages 55-67.
It is proposed therein to consider the channel coding and the modulation as an integral whole, and to that end to employ channel coding combined with digital modulation. This renders it possible to increase the efficiency of the digital transmission and thus to improve performance without reducing spectral efficiency. The redundancy added by the coding is transmitted as a redundancy of the alphabet instead of by reducing the data rate. This technique is based on the maximization principle of the minimum euclidian distance between sequences of transmitted encoded points.
Thus, after a coding operation which converts p information bits into m bits, wherein m&gt;p and m-p represents the redundancy added for the protection of information components during transmission, using a 2.sup.m -stage modulation, there are 2.sup.m-p stages to transmit this redundancy. This modulation technique allows the use of a spatial distribution instead of a temporal distribution of the redundancy.
Following the discovery by G. UNGERBROECK of trellis coded modulations (TCM), combinations of block-wise coded modulations (BCM) and trellis coded multidimensional modulations have been proposed.
TCM's of a moderate complexity (4 or 8 states) can yield a coding gain of 3 to 4 dB. But in large-scale applications the incorporation of a Viterbi decoder, which is required to decode these TCM's, remains expensive in the present state of the art. A coding technique which is attractive for these applications is multilevel coding. The significance of this technique is that it is adapted to a simple sub-optimal decoding method which is effected in stages, and offers a good compromise between the performance and the complexity of the structure.
Based on the constellation partitioning principle described by G. UNGERBROECK, the use of multilevel coding has been analyzed more specifically by G. J. POTTIE and D. P. TAYLOR in "Multilevel Codes Based on Partitioning" IEEE Trans. Information Theor. vol. 35, no. 1, Jan. 1989, pages 87-98.
In that article the authors first analyze the multilevel coding principle, consisting of partitioning a modulation constellation and coding the points of the constellation, and secondly describe a multistage coder having a coding stage assigned to each partitioning level, the points being transmitted block-wise via a transmission channel.
On reception, a multistage decoder effects the inverse operation and recovers constellation points corresponding to the transmitted points. In a conventional decoder this triggers decision operations which estimate points and determine code bits of the estimated points as a function of the detected phase and the amplitude of each point received. Depending on various transmission and receiving conditions, however, a certain number of the estimated bits will be faulty. A first stage of the multistage decoder decides on estimated points according to the first level of the partition. The result supplied by this first stage is employed for validating the decoding of the second stage, and so on until the last stage. In the article by G. J. POTTIE and D. P. TAYLOR, an additional bit erasure operation is introduced after each stage, the decoding effecting a concatenated decoding. This improves the performance of the decoder but at the expense of increased complexity. Actually, this requires the addition of a second decoder to correct and accomplish the bit erasure.
The idea of bit erasure has already been described in another connection by R. H. DENG and D. J. COSTELLO in the article "High rate concatenated coding systems using bandwidth efficient trellis inner codes", IEEE Transactions on Communication, vol. 37, no. 5, May 1989, pages 420-427. Although the term concatenated is used in this article, it actually relates to cascaded codes. To concatenate or cascade two codes an external coder is actually used followed by an internal coder. For concatenated codes, each symbol of the outer code is successively coded by the internal code. For cascaded codes, the internal coder encodes a packet of several symbols of the outer code. These two coding operations require on decoding totally different decoding techniques. Actually, in the case of decoding of concatenated codes, each symbol after decoding is independent of the symbol following it. In contrast thereto, when decoding cascaded codes, the symbols after the decoding operation are dependent and must be processed accordingly. More specifically, coding by cascaded codes requires an interlacer on coding and a deinterlacer on decoding, between the internal and external coder/decoder. This will obviously render the processing operation and hardware realization of the coding and decoding devices more complex. Moreover, as indicated by the aforesaid article, the coding gain obtained, as compared to the technique without erasure, is between 0.05 and 0.1 dB, which is relatively moderate.